Core Data: Folder like functionality - ios

Question

Im trying to make a entity in core data that acts like a folder, like on a mac basically where you can put a object inside, and another folder. currently i have folders, and the individual bookmark (another entity) and bookmarks can be in the folder, but i cant get Folders to go inside another folder.
Here is my NSFetchedResultsController,
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Bookmark" inManagedObjectContext:self.context];
[fetchRequest setEntity:entity];
[NSFetchedResultsController deleteCacheWithName:#"Folder"];
NSPredicate *pred = [NSPredicate predicateWithFormat:#"folder == %#", self.folder];
[fetchRequest setPredicate:pred];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"title" ascending:YES];
NSArray *sortDescriptors = #[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.context sectionNameKeyPath:nil cacheName:#"Folder"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
return _fetchedResultsController;
}
if the NSEntityDescription Entity was NSEntityDescription *entity = [NSEntityDescription entityForName:#"Folder" inManagedObjectContext:self.context]; it would crash with the error,
* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'keypath folder not found in entity '.
How can i fix this? i am stumped at this issue.
The Folder entity has a to-many relationship with the bookmark entity, the bookmark entity has the following attributes, title, url. Folder has the following attributes, title, displayOrder, isFolder.

Answer 1

NSPredicate *pred = [NSPredicate predicateWithFormat:#"folder == %#", self.folder];
This line is fine when your entity is a bookmark. If you just change the entity to Folder, and leave this predicate in place, then the fetch request will look for the key folder on a Folder entity, which doesn't exist according to your data model description above. If you want to fetch all folders, don't set a predicate.
To allow folders to contain folders, you need to define a relationship from the Folder entity to itself, something like this:
Here the Folder entity has two relationships, a to-many relationship called subfolders, and its inverse, a to-one relationship called parentFolder. The top level folder has nil for its parentFolder.
When fetching the folders contained in a folder, you either simply use the set returned by folder.subfolders, or a fetch request predicate where "parentFolder == %#",folder.